The reception of traffic in point to multipoint (PTM) communication is typically organized in the following way:
A potential receiver must become a member of a receiver group (join the group) and whenever he wants to quit the reception he must leave the group. During the group membership the PTM data reception is possible. Parties not being members do not receive this data. Joining and leaving can be done whenever wanted. This kind of communication model is called multicasting. A special case of multicasting is broadcasting in which the data is delivered to all potential receivers.
When multicast communication model is applied to a mobile environment, limiting the data reception only to joined members might become a problem since the data is delivered over a radio link thus making eavesdropping possible to non-group members. With encryption this problem can be solved by enabling only authorized parties (i.e. receivers that have joined the group) to decrypt the delivered encrypted data. The decryption is made possible by giving the decryption key only to the group members.
The work to standardise the Multicast as a new service has been started in 3GPP (Third Generation Partnership Project). The aim in this work is to enhance the current capabilities in UTRAN (UMTS terrestrial radio access network) (and maybe later also in CN (core network)) the way that it is also capable of providing such services, which are using the common network resources, but which are intended only to a restricted group of people in a cell. These requirements are not fulfilled in current Cell Broadcast concept, which is already standardised in 3GPP release 99.
Basically the standardisation of the multicast type of service means that the new service concept should be capable of transmitting data simultaneously to a group of people, who previously indicated their interest to receive data from a Multicast service. As part of their indication they also accepted that the service provider is allowed to charge subscribers for the service (the charging can be based on e.g. monthly fee, the usage time of the multicast service or the amount of received data). It is noted that the service provider can be either the external service provider (e.g. a person, community, state, government, company) who does not own the network or operator itself, who owns the network through which multicast data is transmitted.
In one cell the multicast related data is sent at the same time to all subscribers by using a single communication path on the radio interface. In UTRAN this communication path can consist of e.g. SCCPCH (Secondary control Channel, a physical channel), which is currently used to transmit data from common channels and the FACH (Forward Link Access Channel, a transport channel), which is devoted for the cell broadcast services. The main requirement for the used channel is that this channel can/is allowed to listen more than one UE (User Equipment) and it is capable of transmitting also streaming type of data.
A cell broadcast service is a service type, which is already part of the 3GPP release 99. Cell broadcast service uses as a transport channel Forward Access Channel (FACH) and on the air interface secondary common physical channel (SCCPCH). The cell broadcast service is characterised by such services, which are not secured or charged from end users by the service provider (or operators) and each UE in the cell—even if they are in Idle mode—are allowed to listen the data from the air interface, which is belonging to the cell broadcast service. A typical cell broadcast service could be e.g. small advertisements, road information etc.
In order to use such a commonly known channel on the air interface and at the same time to provide e.g. charging, in the multicast scheme it should be possible for the service providers (or the network) to allow only the authorised subscribers to access the multicast service. This means that it shall be possible to exclude all unauthorised users from the service even if the UE (User Entity) is capable of listening to the physical channel. For multicast services it has been proposed to use ciphering for this purpose.
Ciphering of multicast services is not a similar concept as the ciphering that is used for e.g. dedicated channels. When the used transport channel is a dedicated transport channel (or a common channel for DCCH (Dedicated control Channel) or DTCH (Dedicated Traffic Channel) (DCCH and DTCH are both logical channels)), the used security information is sent to the UE upon establishment of the radio bearer (RB). For that purpose, for the UE, the NW (Network) has before the actual data transmission (and also upon that) setup separate signalling connections for the transmission of L3 signaling messages. It is noted that L3 stands for Layer 3 (in UTRAN=RRC (Radio Resource Control)), a protocol layer.
The multicast services were not supported by 3GPP rel.99 or rel.4, and therefore no security procedures for point-to-multipoint services have been defined. Also it is not possible to separate between unauthorised and authorised users from service point of view on such channels, which are commonly used for multiple UEs.
It is noted that the sharing of the common channels between multiple UEs is possible due to use of UE specific identification in the data message. This method however is not feasible as such in this case because data is meant to a group of UEs and the use of “group id” instead of UE specific id does not prevent unauthorised UEs to fetching data from the shared common channel.